1. Field of the Invention
This invention relates to improvements in cartons made out of plastics material. More particularly the invention relates to locking joins between adjoining pieces of plastic material and to apparatus for forming these joins.
2. Acknowledgement of Prior Art
Cardboard boxes have traditionally been made from blanks which are folded to form the box. Usually, there is at least one join between the material from which the box is formed on a side wall, possibly near a corner. The bottom of such a box is frequently made from in-turned flaps which are interlocked or sealed together. For cardboard cartons, gluing has been a traditional and satisfactory way of forming the necessary seals.
As environmental concerns have become of more importance and recycling of cardboard materials has been found to be desirable, the presence of glue has become a disadvantage.
The environmental concerns have caused consideration of the use of plastics material cartons. Such cartons, if satisfactory, may be capable of a considerable amount of reuse before recycling is necessary. When recycling becomes necessary, the recovered plastics material is potentially more useful than the recovered recycled material from paper recycling. Theoretically at least, it is possible to recycle plastics material in very pure form, and it may be used for primary uses of the plastic. It is, however, necessary that the plastics material to be recycled must be very pure, i.e. free of contaminants such as glue, metal fasteners, adhesive tape, etc. A certain amount of printing ink is acceptable, but even this will affect the colour of the recycled plastic material.
The use of glue, metal fasteners, adhesive tape or other sealing means for making seals between pieces of plastics material may be undesirable for other reasons. There may be a tendency for metal fasteners to tear out of the plastics material, and it is of course necessary to provide a manufacturing step for inserting them into position. Glues have to be carefully chosen to be suitable for forming permanent seals. Adhesive tapes may be unsightly and uneconomic.
The method of making permanent seals between pieces of plastics materials which has been widely used is that of welding the two pieces of plastic together, either by heat sealing or by ultrasonic welding. This manner of sealing may be satisfactory if accurately carried out over large enough contact areas. Nevertheless, a problem has existed in forming a permanent join between two pieces of plastics material; and the need for simple mechanical means which may be automated for providing such a joint is considerable.
The present applicant has several earlier patent applications directed to cartons made of plastics material. His U.S. patent application nos. 802,292 filed 4th Dec. 1991 and 802,293 also filed 4th Dec. 1991 are directed to openable and closeable seals, which may be used for releasably engaging either or both of the top or bottom flaps of cartons such as beer cartons. Those releasable seals have considerable strength and a variety of advantages.
The present applicant has now directed his attention to the problem of providing a truly permanent seal by simple mechanical means without the need of contaminants for the plastic or without the need for welding.